1. Field of the Invention
The present invention relates generally to the field of hot-water space heating systems, and in particular to a system that recaptures heat contained in exhaust vapor.
2. Description of the Related Art
Due to the high cost of fuel, a strong economic incentive exists to maximize the efficiency of space heating systems. The water vapor produced by the gas or fuel burners employed in typical systems provides a major pathway for energy loss due to the high heat capacity and heat of vaporization of water. Although water vapor is a significant combustion product in these systems, it escapes as flue gas in conventional recirculatory heating systems because the heat exchangers respond primarily to the sensible heat of the burners; therefore, the heated water vapor can contribute only minimally to reheating of the incoming heat transfer medium. Indeed, it has been found that the water vapor in typical flue gas represents approximately 10% of the total available heat from combustion.
An ideal system for reclaiming this energy would cool the water vapor to condensation, thereby capturing the bulk of the heat stored in the vapor. However, practical constraints limit the feasibility of designing such systems. Depending on the amount of excess air used in combustion, the water-vapor dew point of methane or natural gas combustion products ranges from about 120.degree. F. to 138.degree. F. Cooling the flue gas below this temperature range may be practical in a forced-air heating system, where the temperature of the heat transfer medium (heated air) is only about 70.degree. F. However, such cooling is impractical in the case of traditional forced hot-water heating systems. The water-supply temperature in such systems can exceed 200.degree. F., while return water temperatures generally exceed 145.degree. F. and can reach about 180.degree. F.
Colder return temperatures in forced hot-water systems can theoretically be achieved by reducing the water flow rate, so that heat exchange at the building radiators is allowed to progress further. However, flow rates sufficiently slow to reduce return temperatures below the flue gas dew point would result in poor heat delivery through conventional radiators. Similarly, reduction of the supply water temperature would substantially reduce the heating capacity of the system.
One feasible approach to energy recapture in forced hot-water water heating system involves transferring both sensible and latent heat contained in the flue gas to the incoming air stream, rather than to the supply or return water. The resultant introduction at the point of combustion of air having elevated temperature and water-vapor levels reduces the amount of fuel necessary to achieve a given net heating duty, and increases the temperature at which heat exchange from the condensation of water vapor can be achieved. Such systems have been known in the prior art for some time; see, e.g., U.S. Pat. No. 1,291,175 (issued Jan. 14, 1919, and hereinafter referred to as the "'175 Patent"); U.K. Patent No. 2,103,510 (hereinafter referred to as the "'510 Patent"). However, these prior art systems present a number of disadvantages. Multiple pumps and/or elevational requirements for the secondary heat exchanger limit the net energy recovery, as well as causing significant practical constraints on the efficient and economic arrangements of system components. Furthermore, the designs of the secondary heat exchangers found in the prior art exhibit undesirable thermodynamic characteristics and require flow and liquid-level balancing components that further degrade performance.